1. CC112x Feedback to PLL - Extending RX filter BW without increasing noise BW

2. Signal BW and RX Filter BW (1)
Assume 5 kbps and ±2.5 kHz frequency deviation and no frequency offset between RX and TX
What is the signal bandwidth?
What is the minimum required RX filter BW?
Receiver channel filter BW
IF = RF - LO
RF Frequency
Frequency offset
SBW = Symbol Rate + 2·freq dev
LO Frequency

3. Signal BW and RX Filter BW (2)
Assume 5 kbps, ±2.5 kHz frequency deviation, +/-10 ppm crystal tolerance in TX and RX, and 868 MHz operation
What is the signal bandwidth?
What is the minimum required RX filter BW?
What is the theoretical degradation in sensitivity compared to using minimum RX filter BW (no frequency offset)?
Receiver channel filter BW
IF = RF - LO
-X ppm
+X ppm
Wider RX filter bandwidth:
Reduced sensitivity and close-in selectivity
Temperature compensation might not be neccessary
Allows cheaper (i.e. less accurate) crystal to be used
If the RX filter BW changes by a factor X, the theoretical sensitivity changes by 10log(X)
RF Frequency
-2·X ppm
+2·X ppm
Frequency offset
-X ppm
+X ppm
RX BW > SBW + 4·Xppm·RF
LO Frequency

4. Measurement Results
5 kbps, ±2.5 kHz frequency deviation and 12.5 kHz RX filter BW
5 kbps, ±2.5 kHz frequency deviation and 50 kHz RX filter BW
If the RX filter BW changes by a factor X, the theoretical sensitivity changes by 10log(X)
Theory matches measurements in this case!
12.5 kHz
50 kHz
dBm
dBm

5. Crystal Accuracy

6. Feedback to PLL (1)
Extends RX filter BW without increasing noise bandwidth
FREQOFF_CFG = 0x30
RX filter BW extended by ±BW/4
Program 50 kHz Effective BW is 75 kHz
Programmed RX filter BW = noise BW
IF = RF - LO
-X ppm
+X ppm
RF Frequency
Negative Offset
No offset
Positive Offset
Extended RX filter
-X ppm
+X ppm
LO Frequency

7. Crystal Accuracy

8. 50 kHz
No feedback
to PLL
50 kHz and
feedback
to PLL

9. Feedback to PLL (2)
Assume 5 kbps, ±2.5 kHz frequency deviation, +/-10 ppm crystal tolerance,868 MHz operation, and using feedback to PLL
What is the minimum required RX filter BW?
What is the theoretical degradation in sensitivity compared to using 12.5 kHz RX filter BW (no offset case)?
What is the theoretical improvement in sensitivity compared to using 50 kHz RX filter BW (no feedback to PLL case)?
12.5 kHz
33.3 kHz
50 kHz
dBm
dBm
dBm

10. 50 kHz
No feedback
to PLL
33.3 kHz and
feedback
to PLL